The Evolution of Populations

AP Chapter 23

What is microevolution?• Changes in the

allele frequencies of a population from generation to generation

Sources of genetic variation

• Mutations – more common in prokaryotes

• Sexual reproduction – more common in eukaryotes

• Can be discrete characters determined by a single gene locus

• Can be a quantitative character – varying along a continuum by more than one gene * most common in populations

Measures of genetic variability

• Average heterozygosity of a population

• Nucleotide variability

• Geographic variations (cline – graded variation in a character across a geographic area, may parallel an environmental gradient)

A cline

About mutations…

• Most occur in somatic cells. Have to be in gametes to be inherited.

• Point mutations that do not change the amino acids or phenotype are harmless

Otherwise mostly harmful• Chromosomal mutations often deleterious• Rates in animals and plants – 1/100,000

genes per generation – so rare• More rapid in prokaryotes and viruses

About variation in sexual reproduction

• Due to reshuffling of alleles in recombination, crossing-over, independent assortment of chromosomes during meiosis, random combination of gametes in fertilization

How can we tell if gene frequencies are changing?

How to determine gene frequencies

• Hardy-Weinberg equation

• Gene pool – all alleles at all the loci present in a population

• If all individuals are homozygous for the same allele, the allele is fixed.

• Otherwise:

p = frequency of dominant alleles

q = frequency of recessive alleles

Therefore, p + q = 1

Hardy-Weinberg equation

• The gene pool of a population will remain constant (no evolving) from generation to generation IF

• No mutations

• Random mating

• Large population

• No natural selection

• No migration

• The probability that two gametes containing the same allele will come together is equal to (p + q)2.

• p2 + 2pq + q2 = 1

• P2 = homozygous dominants

• q2 = homozygous recessives

• 2pq = heterozygous ones

When doing problems:

• ALWAYS TAKE THE SQUARE ROOT OF THE PERCENTAGE OF RECESSIVE TRAITS FIRST

• That will equal “q”

• Subtract that from 1 to find “p”

• Plug in equation

• Example of a Hardy-Weinberg calculation• Species: Sciurus carolinensis (Grey Squirrel) Alleles:

A (dominant; wild type agouti fur) and a (recessive; melanistic black fur)

• We are studying a population of 1000 squirrels. Of these, 60 (60/1000, or 0.06) are melanistic.If each of these melanistic squirrels carries two recessive alleles, we can use this to calculate the expected frequency of q, since q2 is the frequency of the alleles in the homozygous recessive individuals.

• The square root of q2 is equal to q. (duh) In our example, the square root of 0.06 (no. of recessives) = .25.

• Since p + q = 1.0, you can now solve for p • 1 - 0.25 = 0.75 • Our predicted frequencies, based on the assumption that

the squirrel population is in HW equilibrium, are p = 0.75 and q = 0.25

• plug these values into the HW equation to calculate expected relative genotype frequencies:

• (0.75)2 + 2(0.75)(0.25) + (0.25)2

• This means that if our population of 1000 squirrels is in HW equilibrium, then

• p2 = 0.56 - (0.56 x 1000, or 560 squirrels should be AA) • 2pq = 0.38 - (0.38 x 1000, or 380 squirrels should be Aa) • q2 = 0.06 - (0.06 x 1000, or 60 squirrels should be aa) • Notice that these three frequencies add up to 1.0, 100% of

the 1000 squirrels in the population.

• A Toxic Salamander • Western Newt is the vernacular name for the genus Taricha of

which there are three species: torosa, granulosa, and rivularus.  These are toxic salamanders found exclusively in particular regions of California, the western halves of Oregon and Washington, and western costal Canada up through parts of Alaska (3).  Being newts, they are salamanders that spend the majority of their time on land

A Chi Square test can be done to validate your predictions.

Probability of exceeding the critical value

d.f. 0.10 0.05 0.025 0.01 0.001

---------------------------------------------------------------- • 1 2.706 3.841 5.024 6.635 10.828 • 2 4.605 5.991 7.378 9.210 13.816 • 3 6.251 7.815 9.348 11.345 16.266 • 4 7.779 9.488 11.143 13.277 18.467 • 5 9.236 11.070 12.833 15.086 20.515  

How can allele frequencies be altered?

• Natural selection

• Genetic drift

• Gene flow - migration

Gene Flow

Gene flow increases the variability of the gene pool by adding new alleles.

Genetic Drift

• occurs in a small population when some members “drift” off and form a new colony – may not be representative of the original population

• Can lead to loss of alleles in a population or fixation of harmful alleles

Types of genetic drift:

• Bottleneck Effect – some disaster affects the population

• The resulting population may not have the same characteristics as the original.

Genetic DriftBottleneck Effect

• Founder Effect – some members drift off voluntarily – may not be representative of the original population

http://www.youtube.com/watch?v=Q6JEA2olNts

Breast cancer in the Jewish women

• A disproportionate number of Jewish women have the BRCA1 and BRCA2 mutation: Where the odds in the general population are 1 in 450, for Jewish women, the likelihood that they have a mutation is 1 in 40.

• Geneticists attribute this to the founder effect, a theory suggesting that genes in certain isolated communities can be traced back to a small number of "founders" who marry only within the group. Intermarriage normally gets rid of unhealthy genetic mutations, since only the children who inherit the healthy genes survive. When the founders only marry each other, though, those unhealthy genes stick around.

• For Ashkenazi Jews, the founders were a few thousand people who lived in Eastern Europe 500 years ago.

A result of genetic drift

The Founder Effect in Action: Among the Amish, babies with Ellis-Van Creveld Syndrome are born with six fingers

This figure demonstrates 1) the chance of an allele becoming lost from the population is equal to its initial frequency, and 2) the larger the

population size, the slower (weaker) drift is. 

Natural Selection

• Relative fitness – measure of an individual’s contribution to the gene pool by reproduction

• Acts on the phenotype

Jim's genes allow him to live to be 100, but make him, er, unappealing to prospective mates. Joe's genes make him attractive to the ladies, but at a cost: he's not likely to live past 65.Who has the higher evolutionary fitness?

Types of natural selection

• Directional selection – individuals on one end of a phenotypic range are favored

• Disruptive Selection – when environment selects individuals on both extremes

• Stabilizing Selection – favors more intermediate forms, tending to reduce phenotypic variation

Sexual selection

• Selection for a trait that enhances mating

• Can lead to sexual dimorphism (distinction of males and females by secondary sexual characteristics)

Types of sexual selection

• Intrasexual selection – same sex competing for mates

• Intersexual selection – mate choice, females choose “sexier” male

So, what is the significance of the “tusk” of the narwhal?

You got it!

They also use them for fighting! Probably for females!

Type of sexual selection?

Type of sexual selection?

Type of sexual selection?

http://www.pbs.org/wnet/nature/episodes/what-females-want/video-bachelor-geladas-challenge-chewbacca/839/

Preservation of Genetic Variety

• Diploidy (1n + 1n = 2n)

• Balancing selection – maintaining two or more phenotypes in the population

• Heterozygote advantage

• Frequency-dependent selection – phenotype’s reproductive success declines if too many in the population

Balancing selection

A case of frequency-dependent selection

There’s too many

of us already!

Heterozygote Advantage in Sickle Cell Anemia and Malaria

• People with normal hemoglobin are susceptible to death from malaria.

• People with sickle cell disease are susceptible to death from the complications of sickle cell disease.

• People with sickle cell trait, who have one gene for hemoglobin A and one gene for hemoglobin S, have a greater chance of surviving malaria and do not suffer adverse consequences from the hemoglobin S gene.

Neutral variations

• Do not confer a selective advantage or disadvantage

• Ex in noncoding regions of DNA

Natural selection does not result in perfect organisms!

• Many structures co-opted for new situations

• Often many compromises ex:  human knee is amazing in function, but often weak in

structure

• Also chance events affects population’s evolutionary history.

ex:  pregnant female turtle washing ashore a remote island

• The environment changes what is “perfect”

Remember, natural selection…

• can only work with the alleles present in the gene pool

• New alleles cannot be made in response to new environments!

the “perfect” organism

• There are 100 students in a class. 91 did well in the course whereas 9 blew it totally and received a grade of F. Sorry. In the highly unlikely event that these traits are genetic rather than environmental, if these traits involve dominant and recessive alleles, and if the nine (9%) represent the frequency of the homozygous recessive condition, please calculate the following: – The frequency of heterozygous individuals.

•

• You sample 1,000 individuals from a large population for the MN blood group, which can easily be measured since co-dominance is involved (i.e., you can detect the heterozygotes). They are typed accordingly:

• M (MM) 480

• MN (MN) 420

• N (NN) 100

• Calculate the gene frequencies of M and N by COUNTING alleles in the population.